The use of ultrasound to measure the level of contents in tanks and spillways has been known for a considerable period of time, two examples of this type of apparatus being described and claimed in U.S. Pat. No. 4,596,144 (Federal Industries) and UK patent 2,230,608 (Hycontrol).
The basic principles for using ultrasound to measure the depth of tank contents are well known. A pulse of ultrasound is transmitted, from a transmit/receive transducer(s) mounted above the level of the contents, towards the surface of the contents. This pulse is reflected off the surface and the reflected or echo signals are received by the transducer in its receive mode, for subsequent analysis. The time interval between the incident pulse being transmitted and the echo pulse being received is directly proportional to the distance between the transducer and the contents surface. Thus, the accuracy of the level measurement is critically dependent on the measurement of this time interval.
A typical example of a device of this type can be understood with reference to FIGS. 1 to 3. An ultrasound level measurement device 10 is mounted above the surface 11 of a fluid 12 contained in a tank 13. As shown in FIG. 2, pulses of acoustic energy are transmitted from transducer crystal 15 towards the surface 11, typically in 1 sec cycles. These pulses are reflected off the surface 11 and returned to the transducer 15. As is well known, the interval between the time of transmission and the time of receipt is directly proportional to the spacing between the transducer crystal 15 and the surface 11.
In the example shown in FIG. 2, the transmit pulses are generated by electronically switching a reservoir capacitor 17 using a microprocessor 18. The output voltage from the capacitor 17 is then increased using a step-up transformer 20 so that it is at a level suitable to drive the transducer crystal 15. On the receive side, the echo signals are received into an amplifier 22 via a pair of protection diodes (not shown) whose function is to satisfy intrinsic safety requirements by limiting the passage of energy from the transducer crystal to the electronic circuit. In order to compensate for the fact that the sound waves passing between the transducer 15 and the surface 11 will be attenuated by the characteristics of the medium between the surface 11 and the transducer 15, the received signal is subjected to a time-variable gain step at 23. Thereafter the signal is filtered and, in this particular example, the peak of the transmit pulse envelope determined and a threshold applied thereto so that spurious or unwanted echoes can be eliminated from further consideration. The accuracy of this type of device is reliant on consistently detecting the same part of the received echo by setting a threshold at, say, a quarter of the amplitude of the echo size. It follows that any change in the shape of the echo waveform will lead to an inaccuracy in measurement.
A problem that arises with the device described above is that, when the level 11 is close to the transducer 15, the energy of the reflected pulse is high and has been known to saturate the echo processing circuit resulting in incorrect measurement of the echo size and consequential incorrect setting of the echo detection threshold. This, in turn, reduces the accuracy of the time measurement.
The saturation is typically exhibited when the input protection diodes clip the waveform so that it adopts the shape as shown in FIG. 3B as opposed to the desired shape shown in FIG. 3A.
It is an object of the invention to provide a method and/or a device which will address the aforementioned problem; or which will at least provide a novel and useful choice.